3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3GPP to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
FIG. 1 illustrates an exemplary wireless communication system. An eNodeB 104 serves a UE 106. The eNodeB 104 transmits Downlink (DL) transmissions to the UE 106 and the UE 106 transmits Uplink (UL) transmissions to the eNodeB 104.
Wireless communication systems such as LTE systems can be configured for both Time Division Duplex (TDD) operation and Frequency Division Duplex (FDD) operation. In TDD systems, the base stations transmit and receive on the same carrier frequency. UL and DL transmissions are separated in time by designating subframes as either UL subframes or DL subframes. In FDD systems, separate carrier frequencies are used for UL and DL transmissions.
Typically, a transmitted signal in a radio communication system is organized in some form of frame structure, or frame configuration. For example, LTE generally uses ten equally sized subframes 0-9 of length 1 ms per radio frame. In case of TDD, there is generally only a single carrier frequency, and UL and DL transmissions are separated in time. Because the same carrier frequency is used for UL and DL transmission, both the base station and the UEs need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither DL nor UL transmissions occur in order to avoid interference between UL and DL transmissions. For LTE, special subframes provide this guard time. A TDD special subframe is generally split into three parts: a DL part (DwPTS), a guard period (GP), and an UL part (UpPTS). The remaining subframes are allocated either to UL or DL transmission.
There are seven different TDD UL/DL resource allocations in LTE, illustrated in FIG. 2a. Usually a TDD UL/DL configuration provides about 40%-90% resources for DL. In the current LTE specification, the UL/DL configuration in a TDD system is semi-statically configured which means that it is not reconfigured so often. As a result, the UL/DL configuration sometimes does not match the instantaneous traffic demands.
It is envisioned that wireless data traffic will become more and more localized in the future, as most users tend to gather in the so-called hotspots, or in indoor areas, or in residential areas. Often, when users are located in clusters, they tend to generate different UL and DL traffic patterns at different times. As such, a dynamic feature that adjusts the UL and DL resource allocations to instantaneous or short term traffic variations may be needed in local area cells. Faster TDD reconfigurations, hereinafter referred to as dynamic TDD, have shown potential for achieving good performance in both UL and DL, especially at low to medium system load. Dynamic TDD may become a standardized feature in LTE Rel-12. Dynamic TDD systems use the same TDD frame structures as the ones illustrated in FIG. 2a, but allow the TDD configuration to be changed depending on current traffic demands.
Different signaling methods that support dynamic TDD reconfigurations with different time scale are currently being considered. One possible TDD reconfiguration is allocating each subframe as either UL or DL. However, this option poses challenges to operations such as DL/UL switching, random access, radio link monitoring, and handover. Moreover, this option also makes it impossible to maintain backward compatibility with legacy UEs. A more practical solution is to designate a subset of subframes for dynamic TDD reconfiguration. In this case, the subframes can be divided into two types: static subframes and flexible subframes. The static subframes have fixed link directions, UL or DL, while flexible subframes can be dynamically assigned as either UL or DL.
When dynamic TDD is configured, in general, there are two TDD UL/DL reference configurations, one for UL and one for DL. The TDD UL reference configuration is broadcasted in System Information Block 1 (SIB1) and will be used for legacy UEs. Based on the two TDD reference configurations, some subframes may be used as flexible subframes where either DL or UL can be configured.
One area of concern with dynamic TDD is Hybrid Automatic Repeat Request (HARQ) timing. A HARQ feedback timing is associated with each DL subframe. The association determines when to transmit HARQ feedback for a transmission received in the DL subframe. The association is TDD configuration dependent.
Furthermore, with flexible subframes it may be difficult for a UE to determine when to monitor DL control channels and when to perform DL CSI measurements. A UE may decide to monitor every flexible subframe that has not been designated for UL transmissions. This may turn out to be unnecessary and would lead to heavy power consumption and false detection of a non-existing assignment.